1. Field of the Invention
The field of the invention is central processing units (CPUs) and more particularly to powerdown of such CPUs.
2. Related Art
One of the important features of integrated circuits deigned for portable applications is their ability to efficiently utilize the limited capacity of the battery powered voltage source. Typical applications include cellular telephones and personal digital assistants (PDAs), which might have a Lithium ion battery of 800 mA-Hr capacity or two AAA alkaline batteries as the voltage source. Users expect as much as three to four weeks of standby operation using these devices. Standby operation is when the cellular phone is powered on but not actively involved in a call. Industry estimates are that the integrated circuit is only performing useful work approximately 2% of the time while the phone is in standby mode.
Integrated circuit designers have long used Complementary MOS (CMOS) transistor logic to implement battery powered integrated circuits because the power consumed by the circuit was directly proportional to the switching activity of the circuit as defined in the following equation:Ptotal=Pleakage+CV2F Until recently, the high input impedance of CMOS devices kept the leakage component small enough that it could be ignored. The active component is determined by the capacitance (C), voltage (V) and frequency (F) of circuit operation. Designers have employed several techniques to reduce active power consumption including disabling the clocks during periods of inactivity, reducing the operating voltage and reducing capacitance using smaller process geometries. These steps are important but only addresses a portion of the power lost during standby.
Recent fabrication process advances have permitted circuits of increasingly finer geometries to be fabricated. While these advances have permitted more circuits to be built in a given silicon area, they have had the undesirable effect of increasing the leakage current due to direct tunneling effects caused by thinner gate oxides and narrower channels. Leakage currents have increased from <1 picoamp per micron of gate length in a 1.0 micron feature size process to as much as 1 nanoamp per micron in today's 0.13 micron processes. The increases in leakage current no longer permit the leakage component of the power equation to be ignored.
Removing the power supply from selected circuits during standby is a well known technique employed by board level designers for battery powered applications such as notebook computers. It has similarly been applied to integrated circuits but only to blocks outside of the central processing unit (CPU). A primary reason for not applying this technique to CPUs, has been the difficulty in being able to retain the current processor state information necessary to continue execution after coming out of the standby mode. Notebook computer designers have gotten around this limitation by saving the current processor state information to external storage mechanisms such as a hard disk drive. In such a case there is the overhead required in transferring the state to and from the external storage mechanism. Even if the battery powered device had a hard disk drive, and many don't, the time consuming state transfer would not meet the real time response requirements of the application.
Thus, there is a need for powering down a CPU for reduced standby power consumption while retaining the integrity of the operating state.